1. Field of the Invention
The present invention relates to a color solid-state image pickup device.
2. Description of the Related Art
In relation to a CCD (charge-coupled device) semiconductor solid-state image pickup element and a CMOS semiconductor solid-state image pickup element, as described in, e.g., U.S. Pat. No. 3,971,065, which will be described below, color filters having different spectral transmission factors are stacked on a plurality of photodiodes arranged in a two-dimensional array, thereby enabling pickup of a color image.
Two types of color filters are available: namely, red (R), green (G), and blue (B) color filters of a primary color system; and color filters of a complementary color system which permit passage of light of colors complementary to R, G, and B.
In the color filters of primary color system, for instance, a B filter primarily permits passage of only light having a short wavelength of 470 nm or less. Hence, a photodiode of a light-receiving section with the B filter stacked thereon has sensitivity to B incident light. However, the B filter blocks light having other wavelength components (e.g., G and R), and hence G and R wavelength components that have entered the B filter are not subjected to photoelectric conversion. Thus, color filters of this type suffer a problem of waste of G and R incident light rays and a failure in effective utilization thereof.
In contrast, in the case of the color filters of the complementary color system—spectral filters for permitting passage of light of wavelengths complementary to the primary color components R, G, and B—the color filters are constituted of a yellow (Ye) filter for permitting passage of G and R components complementary to B, a magenta (Mg) filter for permitting passage of B and R components complementary to G, and a cyan (Cy) filter for permitting passage of B and G color components complementary to R. Of the incident light, light wasted by the color filters of complementary color system becomes smaller in quantity than that wasted as a result of use of the color filters of the primary color system. Specifically, the solid-state image pickup device using the complementary color filters can utilize wavelengths of incident light over a wide range and hence has a characteristic of an increase in sensitivity. For this reason, a video movie camera (for photographing a moving image) which encounters difficulty in utilizing an auxiliary light source, such as a flash, frequently adopts a solid-state image pickup device using complementary color filters.
Meanwhile, in a solid-state image pickup device using color filters of a complementary color system, a signal output from a pixel with a Ye filter stacked thereon becomes a G+R signal; a signal output from a pixel with a Cy filter stacked thereon becomes a G+B signal; and a signal output from a pixel with a Mg filter stacked thereon becomes an R+B signal. Hence, after the G+R, G+B, and R+B signals have been read from the solid-state image pickup device, the signals must be subjected to a color signal separation computation processing performed by an external circuit, thereby extracting R, G, and B signal components.
Specifically, the solid-state image pickup device using the filters of a complementary color system require color signal, separation computation processing. Hence, when compared with a solid-state image pickup device using color filters of a primary color system capable of directly obtaining signals of R, G, and B color components, the solid-state image pickup device using the filters of complementary color system suffers a problem of deterioration of image quality in terms of color reproducibility and noise. Therefore, a still camera placing an emphasis on image quality (i.e., a camera for photographing a still image) frequently adopts a solid-state image pickup device using color filters of a primary color system. Sensitivity is compensated for by means of an auxiliary light source.
“A Planar Silicon Photosensor with an Optimal Spectral Response for Detecting Printed Material” by Paul A. Gary and John G. Linvill, IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-15, No. 1, Jan., 1968. (hereinafter referred to as “Publication 1”) describes dependence of a photoelectric conversion characteristic of a photodiode on the depthwise position of a silicon substrate as well as on the wavelength of incident light.
An example of solid-state color imager comprised of three photo-sensitive layers, to which this idea has been applied is described in U.S. Pat. No. 4,438,455, which will be provided below.
The solid-state color imager with three photo-sensitive layers of U.S. Pat. No. 4,438,455 configured on the principle described in Publication 1 has a structure for extracting signals of three colors; i.e., R, G, and B. Without using color filters over-laid on the photo-sensitive elements, no light absorption of color filter material has arisen, and hence, incident light can be effectively converted into an electric signal.
As shown in FIG. 55 (corresponding to FIG. 3 of U.S. Pat. No. 4,438,455), U.S. Pat. No. 4,438,455 describes a structure 101 embodied by means of superimposing three photo-sensitive layers 102, 103, 104 and changing the depth of each photo-conductive layer against the incident light to apply the principle described in Publication 1 to the above structure.
The other example of CCD and MOS type solid-state color imager to which this idea has been applied is described in JP-A-1-134966, which will be provided below.
The solid-state color imager of JP-A-1-134966 configured on the principle described in Publication 1 has a structure of three story N+P photo-diode with different depth for extracting signals of three colors; i.e., R, G, and B, from one pixel. Without using color filters over-laid on the photo-diode elements, no light absorption of color filter material has arisen, and hence, incident light can be effectively converted into electric signal. Further, false signals or false colors, such as moiré, can be improved.
As shown in FIGS. 56A to 56C (corresponding to FIGS. 1(a) to 1(c) of JP-A-1-134966), JP-A-1-134966 describes a structure embodied by means of changing the depth of each N+P photo-diode to apply the principle described in Publication 1 to the above structure.
As shown in FIG. 56A, short wavelength light such as Blue is detected by the shallow N+P photo-diode 201. Long wavelength light such as Red is detected by the deep N+P photo-diode 203 as shown in FIG. 56C. The medium wavelength light such as Green is detected by the N+P photo-diode 202 locating in the depth of between the above two N+P photo-diodes as shown in FIG. 56B.
Since the solid-state image pickup does not employ color filters, spectra of color component output signals (R, G, B) mutually become larger, thereby causing overlaps. This also presents a problem of difficulty in faithful color reproduction and an attempt to enhance image quality.
In addition, an increase in the number of pixels of a solid-state image pickup device to be incorporated in a recent digital still camera, a video movie camera, or the like has recently been pursued in earnest. Conversely, the area of a light-receiving section accounting for each pixel of the solid-state image pickup device has become smaller. Hence, photographing an image with the same sensitivity as that ever achieved becomes more difficult. Development of a color solid-state image pickup device which can simultaneously achieve enhanced sensitivity, color reproducibility, and a low noise characteristic without involvement of color signal separation computation processing is desired.